During coronary angioplasty a balloon-tipped catheter is advanced to the site of an atherosclerotic lesion. By inflating the balloon the artery is opened and the normal flow of blood restored without having to resort to open heart surgery. Unfortunately, in about one third of patients who undergo angioplasty the artery restenoses (or closes down again) within six months of the procedure. In recent years several novel approaches to the problem of restenosis have been undertaken, including coronary stents. These devices are made of stainless steel and form an expandable scaffold around the angioplasty balloon. When the balloon is inflated at the side of the coronary lesion, the stent also expands and remains in place within the artery to hold it open and prevent restenosis. Preliminary clinical trials with stents have shown promising results. However, there have been some cases of thrombosis (blood clots) within the stent. We were concerned that abnormal flow disturbances induced by the stent could be responsible for such thrombosis. To study this possibility, we built a model heart and used a transparent fluid with the same properties as blood. By mounting electrochemical sensors flush with the walls of our model coronaries, we were able to compare the stability of flow in these arteries before and after the insertion of coronary stents. We discovered that under simulated resting conditions, the stents had no effect on coronary flow patterns. However, under mild exercise conditions, the stents disturbed the flow in their immediate downstream vicinity. If prolonged, the wall shear stress was sufficient in this case to cause platelet aggregation and thrombosis.